This invention relates generally to traffic sensitive digital communications systems; more particularly, it concerns the provision of asynchronous, adaptive codecs in such systems.
U.S. Pat. No. 3,898,387 to Fort discloses an asynchronous digital voice communications systems suitable to switching system application and potentially a ditigal transmission system. The analog to digital conversion mechanism is FM in nature. The system has the great merit of adapting itself automatically to the traffic imposed on the system with degradation of quality of the signal resulting during high traffic periods rather than physical space blocking normally encountered in most switching systems. Chapter 7 of the text, "Delta Modulation Systems" published by Halsted Press (Wiley) by R. Steele discusses several kinds of syllabically companded delta modulation A/D converters which are very efficient regarding the effective channel bit rate required to accommodate the wide dynamic range of the signals required in telephone networks. These continuously variable slope delta modulation (CVSDM) codecs usually operate synchronously and are therefore suitable for multiplexing purposes in multi-channel communication systems. Chapter 6 of that text discusses the possibility of asynchronous delta modulators which would be suitable for the interface of the system described in the above referenced Fort patent. There are three examples of such asynchronous delta modulators, namely: FIGS. 6.l, 6.4, 6.7. Each has its own advantages and disadvantages depending upon application but all are linear DM codecs (asynchronous). They are, therefore, not efficient as regards channel bit rate under the conditions of wide dynamic range of the signal to be encoded.